In a cellular communication system, when there are services to be transmitted between two User Equipments (UEs), service data to be transmitted from a UE1 to a UE2 is first transmitted to the base station (or referred to as a Node B or an evolved Node B or an enhanced Node B, NB or eNB for short) of the cell where the UE1 is located via an air interface, then the base station transmits the user data to the base station of the cell where the UE2 is located via a core network, and the base station of the cell where the UE2 is located transmits the service data to the UE2 via an air interface. The service data transmission from the UE2 to the UE1 is realized through a similar processing procedure. As shown in FIG. 1, when the UE1 and the UE2 are located in the same cellular cell, although the two UEs are covered by the cell of the same base station, the data transmitted between the two UEs still needs to be forwarded by a core network, and two parts of wireless spectrum resources are consumed in a single data transmission process.
Thus, the foregoing cellular method is not the optimal choice when user equipments 1 and 2 are located in the same cell and adjacent to each other. Actually, as mobile communication services are increasingly diversified, for example, as the popularization of social networks and electronic payment in wireless communication systems leads to an increasing demand for the service transmission between adjacent users. Thus, a Device-to-Device (D2D) communication mode is getting more and more attention. As shown in FIG. 2, D2D communication refers to the direct transmission of service data from a source UE to a target UE via an air interface, without forwarding the service data using a base station or a core network. For users communicating with each at a near distance, D2D communication not only saves wireless spectrum resources but also reduces the data transmission workload of a core network.
In cellular communication, when two UEs communicate with each other, generally, the two UEs are connected by network equipment/equipments (e.g. a base station or a core network equipment) with each UE not knowing the position of the other UE. Nonetheless, D2D communication is established under the premise of the mutual discovery of UEs. However, no effective solutions have been proposed to address the problem existing in the conventional art regarding how to enable a UE to discover an adjacent UE (the discovery is also called peer discovery).